This year’s flu season could be particularly severe, as it appears to be dominated by strains of flu virus that are not fully covered by this year’s vaccine. However, flu shots and nasal sprays are still important preventive steps recommended for everyone, especially those at high risk.

This is the message the Centers for Disease Control and Prevention (CDC) is sending out this week, as they urge “immediate vaccination for anyone still unvaccinated this season,” and recommend “prompt treatment with antiviral drugs for people at high risk of complications who develop flu.”

The federal agency says the most common strains of flu viruses that have been circulating this year are so-called “drift variants” of A H3N2 viruses that are genetically different from the strains used to make this season’s vaccine. The seasonal flu vaccine is formulated to protect against three or four different flu viruses, generally identified by analyzing the most dominant strains from the previous year.

Health officials point out, though, that even during a season when the vaccine only partially protects against one flu virus, it can still protect against the others. They also note that it will likely offer protection against other flu viruses that may become more common later in the season.

‘Drift strains’ of flu virus breach vaccine’s protection

The reason that this year’s flu vaccine may not cover all the strains that end up in circulation is because the time it takes to make a new season’s flu vaccine is longer than the time it takes for flu viruses to mutate into new variants.

Flu vaccine scientists – like long-range weather forecasters – are tasked with making predictions far in advance of the flu season and in the meantime, things can change. It doesn’t happen every year, but sometimes tiny changes in virus genes can accumulate in a few months to cause it to drift outside the range the scientists have anticipated – hence the name ‘drift variants’.

This year, the World Health Organization (WHO) made its recommendations for 2014-2015 Northern Hemisphere vaccine in mid-February. This kicked off the vaccine production cycle, which takes around 6 months to prepare the millions of doses needed.

At that time, only a very small number of drifted H3N2 viruses had been found among the thousands of samples that had been collected and tested for analysis by WHO scientists.

However, more recent surveillance indicates that so far, seasonal influenza A H3N2 viruses have been most common. This is concerning, says the CDC, because in previous years, H3N2 strains have tended to result in more severe illness and send more people to the hospital — and cause more deaths, especially among the elderly, children and people with other health problems.

H3N2 viruses predominated during the flu seasons of 2003-2004, 2007-2008 and 2012-2013, when the largest number of deaths were recorded over the past decade. The flu is still at low levels in the United States, but 1,228 confirmed cases were reported to the CDC during the week that ended Nov. 22 and five children have already died from the virus.

“The flu is unpredictable,” says CDC Director Dr. Tom Frieden. This is “likely to be an H3N2 predominant season … and H3N2 predominant seasons tend to have more hospitalizations and more deaths.”

Around half of H3N2 viruses analyzed so far are drift variants

Besides the severity of the strains, health officials are also alarmed at the discovery that around half of the H3N2 viruses analyzed so far are drift variants, suggesting the vaccine’s ability to protect against H3N2 viruses may be reduced. However, even if they do become infected by these drift strains, people are likely to experience a milder illness if they are vaccinated.

For example, when an H3N2 drift variant dominated the 2007-2008 flu season, the vaccine was still shown to be 42 percent effective against H3N2 viruses compared with an overall efficacy of 37 percent.

“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said Dr. Frieden. He warned that, despite low flu activity at the moment, there are signs that flu cases are already starting to pick up in some regions. However, the season is only just beginning and, “It’s not too late to get your vaccine,” he added.

Besides getting vaccinated, Dr. Frieden pointed to two other important steps that should be taken to save lives during any flu season. Firstly, those at high risk of complications should receive prompt treatment if they fall ill, and secondly – to prevent spreading flu virus – people should stay home when they get sick.

Flu vaccine recommended for everyone over 6 months old

The CDC recommends that everyone – with rare exceptions – over the age of 6 months should have the seasonal flu vaccine.

People at high risk of seasonal influenza include children under the age of 5, adults aged 65 and over, pregnant women, and people with certain health conditions such as asthma, diabetes, kidney, heart or lung disease.

The CDC urges that people at high risk should seek medical advice as soon as they start with flu symptoms, which can include sore throat, runny or stuffy nose, fever, cough, body aches, headache, chills and fatigue. There is evidence that antiviral drugs against flu are more effective when started in the first 48 hours following the start of symptoms, the CDC notes.

According to the WHO, as of November 27, 2014, global flu activity remained relatively low, with the exception of some Pacific Islands. In the northern hemisphere, annual flu epidemics occur during autumn and winter – generally peaking sometime around February – and affect up to 15 percent of the population, the WHO says.